1. Field of the Invention
The present invention relates to a negative resist composition suitably usable for the ultramicrolithography process such as production of VLSI or a high-capacity microchip or in other fabrication processes, and a pattern forming method using the same. More specifically, the present invention relates to a negative resist composition capable of forming a high-resolution pattern by using electron beam or X-ray, and a pattern forming method using the same. That is, the present invention relates to a negative resist composition for use in the process using a substrate having thereon a specific underlying film.
2. Description of the Related Art
In the process of producing a semiconductor device such as IC and LSI, fine processing by lithography using a photoresist composition has been conventionally performed. Recently, the integration degree of an integrated circuit is becoming higher and formation of an ultrafine pattern in the sub-micron or quarter-micron region is required. To cope with this requirement, the exposure wavelength also tends to become shorter, for example, from g line to i line or further to KrF excimer laser light. At present, other than the excimer laser light, development of lithography using electron beam or X-ray is proceeding.
In particular, the electron beam lithography is positioned as a next-generation or next-next-generation pattern formation technique and a negative resist assured of high sensitivity and high resolution is being demanded.
Also, the electron beam lithography is widely used for the preparation of a photomask employed in the semiconductor exposure because of its high resolution property. The process of preparing a photomask is as follows. A shielding layer mainly comprising a shielding material such as chromium is provided on a transparent substrate such as glass substrate to prepare a shielding substrate, and a resist layer is formed thereon and selectively exposed to form a resist pattern on the shielding layer. Subsequently, the shielding layer in the portion where the pattern is not formed is etched using the resist pattern as a mask to transfer the pattern to the shielding layer, whereby a photomask comprising a transparent substrate having provided thereon a shielding layer in a predetermined pattern can be obtained.
In the processing using an electron beam, which is a different system from the block exposure of light, elevation of sensitivity is very important for reducing the processing time, but in the case of a negative resist for use with an electron beam, when higher sensitivity is pursued, the line edge roughness is worsened in addition to reduction of the resolution and deterioration of the pattern profile, and a resist capable of satisfying these properties all at the same time is strongly demanded. The line edge roughness as used herein means that the resist edge at the interface between the pattern and the substrate irregularly fluctuates in the direction perpendicular to the line direction due to the resist characteristics and when the pattern is viewed from right above, the edge gives an uneven appearance. This unevenness is transferred by the etching step using the resist as a mask and impairs the dimensional precision. Particularly, in the ultrafine region of 0.25 μm or less, the line edge roughness is a very important problem to be solved.
Also, it is known that when a resist pattern is formed on the shielding layer used for the preparation of a photomask, deterioration of the pattern profile is brought about. In particular, when a negative resist is used, pattern collapse occurs due to erosion at the interface with the substrate, giving rise to an issue that the resolving power significantly deteriorates, and this becomes a problem.
The high sensitivity is in a trade-off relationship with high resolution, good pattern profile and good line edge roughness and it is very important how to satisfy these properties all at the same time.
As regards the resist suitable for the electron beam or X-ray lithography process, a chemical amplification-type resist utilizing an acid catalytic reaction is mainly used in view of high sensitivity and in the case of a negative resist, a chemical amplification-type resist composition mainly comprising an alkali-soluble resin, a crosslinking agent, an acid generator and an additive is being effectively used.
Various studies have been heretofore made to enhance the performance of the chemical amplification-type negative resist. The following studies have been made on the additive, particularly, the ammonium salt-type additive. For example, JP-A-4-51243 discloses a combination of a tetraalkylammonium salt and a novolak resin, JP-A-8-110638 discloses a trialkylammonium hydroxide, and JP-A-11-149159 discloses a combination of a tetraalkylammoinum salt and a polymer having a carboxylic acid in the side chain.
However, it is impossible by any combination of these conventionally known compounds to satisfy high sensitivity, high resolution, good pattern profile, good line edge roughness, and good in-vacuum PED characteristic in an ultrafine region all at the same time.
Also, in JP-A-10-186660, use of an organic carboxylic acid is studied, but use of a specific resin and a specific ammonium salt is not described. Furthermore, JPA-2003-295439 sets forth use of an ammonium salt but is silent on use of an organic carboxylic acid. In addition, neither of these patent publications refers to improving the line edge roughness while giving a good pattern profile with less erosion, which is the effect of the present invention.